Some anonymous source has indicated that USAC was clearly superior to HE-AAC and it did very good on speech though something like "USAC at 48 kpbs = HE-AAC at 64 kbps" shouldn't be expected. It's more like 20-25% of bitrate gain at 20-32 kbps and 10-12% at 64 kbps. That's still pretty good.

There is no information if USAC will be used at higher bitrates as ( LC-AAC's area). The core coder of USAC includes a new enhanced SBR (eSBR). Both SBR and new eSBR have no advantage over LC-AAC at bitrates higher than 80 kbps. It’s unclear if eSBR can be disabled at higher bitrates.

It will be interesting to hear opinions and if somebody knows something more about it.